Solder balls are used as electrical interconnects in ball grid array (BGA) packages. In a BGA package, an array of solder balls is used to couple a semiconductor die to a circuit substrate such as a printed circuit board (PCB).
A method that can be used to form a BGA package is the solder ball attach process. In a solder ball attach process, solder balls are mechanically placed on a ball land pattern of conductive regions in a semiconductor carrier or substrate. After the solder balls are placed on the specified ball land pattern, the semiconductor die is flipped over and is mounted on a circuit substrate.
The solder balls typically have a lead-based solder alloy. For example, the solder alloy may be a near eutectic tin-lead (Sn—Pb) alloy that melts at about 183° C. When the solder balls are present between the semiconductor die and the circuit substrate, the balls are heated to a temperature at or below the melting temperature of the solder alloy. The purpose of heating the balls below melting point is to ensure that the solder balls do not collapse. The solder balls are fluxed and re-flowed to join the circuit substrate to the semiconductor die. During fluxing, oxides in the solder are removed and the conductive surfaces of the semiconductor die and the circuit substrate are wet with the melting solder. After fluxing and re-flow, solder interconnects are formed between the semiconductor die and the circuit substrate. The formed solder interconnects electrically couple the circuit substrate and the semiconductor die together.
FIG. 1 shows a semiconductor die package. In the die package, a carrier 100 is provided with a rectangular cavity 100-1 that receives a semiconductor die 102. In this example, the semiconductor die comprises a vertical metal oxide field effect transistor (MOSFET) and is in a Ball Grid Array-type package (BGA). An array of solder balls 108 is on the surface of semiconductor die 102 as well as on an edge surface 106 of the carrier 100. The solder ball array 108 is divided into two groups. A first outer array of solder balls 108-2 connects to the carrier edge surface 106 and an internal array of solder balls 108-1 connects to the die surface. The array of solder balls 108 can be mounted and coupled to a circuit substrate such as a circuit board.
The outer array of solder balls 108-2 provides the connection to the drain terminal of the MOSFET, while the inner array of solder balls 108-1 provides the connection to the source and gate terminals of the MOSFET. A corner solder ball 108-3 can be dedicated for the gate terminal and the remaining solder balls 108-1 in the inner array 108-1 provide for a distributed, low resistance connection to the source terminal of the BGA MOSFET.
Solder ball technology could be enhanced and improved to provide for a more robust design. For example, the strength of adhesion of the ball interconnects could be improved. Solder interconnects formed using a ball attach process can break if they are weak. If one or more solder interconnects fail in a die package, the entire package can be inoperative. In addition, during processing (e.g., during reflow), the solder balls deform. The deformation can cause the solder balls in the array to have varying heights. As a result, the ends of the solder balls may not be coplanar with each other. If, for example, some of the solder balls on an array of conductive pads are taller than other solder balls in the array, the shorter solder balls may not make contact with both the semiconductor die and the circuit substrate. The formed die package could be inoperative due to the faulty solder interconnects. Also, solder balls can move during processing. If the solder balls move from their intended locations, the desired interconnections may not be formed in the semiconductor die package. Lastly, many solder balls contain lead. Lead is not an environmentally friendly substance. It would be desirable to reduce if not to eliminate the amount of lead used in a semiconductor die package.
Embodiments of the invention address these and other problems.